Problem 21 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Correct name for \(\left(\mathrm{C}_{2} \mathrm{H}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\) (a) 4,4 -Diethyl-3-methyl-3-butenoic acid (b) 4-Ethyl-3-methyl-3-hexenoic acid (c) 3-Ethyl-4-methyl-3-hexenoic acid (d) 3-Ethyl-4-methyl-3-hexen-6-oic acid

Step-by-Step Solution

Answer

The correct name is 3-Ethyl-4-methyl-3-hexenoic acid.

Step 1: Identify the Longest Carbon Chain

The structure given is \((\mathrm{C}_{2} \mathrm{H}_{3})_{2} \mathrm{C}=\mathrm{C}(\mathrm{CH}_{3}) \mathrm{CH}_{2}\mathrm{CO}_{2} \mathrm{H}\). Identify the longest carbon chain that includes the double bond and carboxylic acid group. This chain has six carbon atoms, so the base name of the compound will be a hexene derivative.

Step 2: Number the Carbon Chain

Number the carbon chain from the end nearest to the carboxylic acid group, assigning the carboxylic acid group the highest priority as it is the most oxidized functional group. The carboxylic carbon is carbon 1.

Step 3: Identify and Name Substituents

Identify the substituents on the chain: Two ethyl groups \((\mathrm{C}_{2} \mathrm{H}_{5})\), hydrogens, and a methyl group \((\mathrm{CH}_{3})\). The ethyl groups are located on carbon 3, and the methyl group is located on carbon 4.

Step 4: Determine the Position of the Double Bond

The double bond in this chain is between the third and fourth carbon atoms. Therefore, it is a 3-hexenoic acid.

Step 5: Assemble the IUPAC Name

Combine all the elements: the base chain (hexene), the position of the double bond (3-), and the substituents with their positions (3-ethyl, 4-methyl). The correct IUPAC name for the compound is 3-Ethyl-4-methyl-3-hexenoic acid.

Key Concepts

Organic Chemistry Naming

Understanding organic chemistry naming, or IUPAC nomenclature, is a vital skill for anyone studying chemistry. The International Union of Pure and Applied Chemistry (IUPAC) has created a set of standardized rules to ensure that each compound has a unique and informative name.

In organic chemistry, the name of a compound typically provides key information about its molecular structure. Here's a simple breakdown of the basic steps:

• Identify the longest chain of carbon atoms, known as the parent hydrocarbon chain. This chain forms the base of the compound's name.
• Identify and name the functional groups attached to this chain. Functional groups are specific groups of atoms that dictate the chemical behavior of the compound.
• Number the carbon atoms in the longest chain, ensuring the highest priority functional group has the lowest possible number. This step is crucial for indicating the positions of double bonds and substituents accurately.
• Combine these elements: the parent chain, functional groups, substituent positions, to form the IUPAC name.

Mastering this method will help make what seems like a complex naming process understandable and straightforward.

Carboxylic Acid

Carboxylic acids are a versatile group of organic compounds characterized by a carboxyl group (\(-CO_2H\)) attached to a carbon chain. This group is incredibly reactive and crucial in chemical synthesis. The typical formula for a carboxylic acid is \(R-COOH\), where \(R\) represents a hydrocarbon chain.

When naming carboxylic acids:

• The carboxylic carbon is always given the number 1 in the carbon chain. This is because carboxyl groups take precedence in nomenclature.
• The suffix "-oic acid" is added to the name of the parent hydrocarbon.
• If there are any substituents, they are accounted for with a number indicating their position on the carbon chain.

The IUPAC names often reveal both the presence and position of each major group. Understanding the structure and priority of the carboxyl group aids in comprehending its prominence in the naming convention.

Double Bonds in Hydrocarbons

Double bonds are a key feature of hydrocarbons, altering both their shape and reactivity. Double bonds are expressed in the compound's name with the suffix "-ene," differentiating them from single bonds, which use "-ane."

When dealing with hydrocarbons that feature double bonds:

• Locate the longest carbon chain that includes the double bond. This chain determines the base name of the compound.
• Provide the position of the double bond by numbering the chain in a way that the double bond receives the lowest possible number.
• In complex compounds, including the position number directly before the suffix "-ene" is common. For example, a double bond between carbons 3 and 4 would be indicated as "3-ene."

This detailed attention to numbering and location makes it clear where the double bonds are situated, which can drastically affect the compound's properties and reactions under different conditions.